The present invention relates to an exhaust-gas aftertreatment device for an internal combustion engine.
European Published Patent Application No. 0 560 991 describes an exhaust-gas aftertreatment device with nitrogen oxide storage catalytic converter, in which, during the lean-burn operating phases, the nitrogen oxides are removed from the exhaust gas from the associated internal combustion engine by being stored in the nitrogen oxide storage catalytic converter. In relatively short, rich-burn operating phases of the internal combustion engine, the nitrogen oxides, which are usually stored in nitrate form in the storage material of the nitrogen oxide storage catalytic converter, are released again and, at the catalytic centers of the catalytic converter, which are preferably formed by precious metals, such as for example platinum, are reacted with the reducing exhaust-gas constituents in the rich exhaust gas to form nitrogen (nitrate regeneration). The recurring alternation between lean-burn and rich-burn operating phases allows effective removal of nitrogen oxides from the exhaust gases from predominantly lean-burn internal combustion engines, such as lean-burn direct injection spark-ignition engines or diesel engines.
Nitrogen oxide storage catalytic converters are gradually deactivated by the uptake of sulphur which is present in the exhaust gas and accumulates in the storage material in the form of sulphates, thus increasingly reducing the ability of the nitrogen oxide storage catalytic converter to store nitrogen oxides over the course of time.
German Published Patent Application No. 198 27 195 describes a method which allows nitrogen oxide storage catalytic converters which have been deactivated in this manner to be regenerated. In methods of this or similar types, the sulphur which is stored in sulphate form is released from the nitrogen oxide storage catalytic converter under reducing conditions and at elevated temperatures, in the form of gaseous compounds (desulphating). It is preferable for the sulphur in sulphate form to be reduced to form sulphur dioxide (SO2). Depending on the amount of sulphur which has built up in the nitrogen oxide storage catalytic converter and on the procedure used for desulphating, however, it is possible that some proportion of the sulphur which is in sulphate form may also be reduced to form gaseous hydrogen sulphide (H2S). Since even the tiniest concentrations of hydrogen sulphide are experienced as an extremely unpleasant smell, the emission of H2S into the environment must be avoided at all costs during desulphating of the nitrogen oxide storage catalytic converters which have been deactivated by sulphur. However, conditions which are similar to those which are deliberately established during a desulphating operation may also occur during particular driving states, for example full-load enrichment, and may then likewise cause undesired emission of H2S into the environment.
German Published Patent Application No. 199 21 974 describes an exhaust-gas aftertreatment device which, in addition to a nitrogen oxide storage catalytic converter, has a particle filter, upstream of which there is an oxidation catalytic converter, the nitrogen oxide storage catalytic converter being arranged downstream of the particle filter in the exhaust-gas aftertreatment device. With the selected arrangement of oxidation catalytic converter, particle filter and nitrogen oxide storage catalytic converter, it is possible for both nitrogen oxides and particles to be effectively removed from the exhaust gas in particular from diesel engines. The oxidation catalytic converter has the function of oxidizing the nitrogen monoxide (NO) contained in the exhaust gas to form nitrogen dioxide (NO2). The NO2 which is formed oxidizes the particles, which predominantly include carbon and have been deposited on the particle filter, even at relatively low temperatures. However, in this case too it is necessary for the nitrogen oxide storage catalytic converter to be subjected to a desulphating procedure from time to time, and consequently, under corresponding operating states or driving states, the problem of H2S being released into the environment persists.
It is an object of the present invention to provide an exhaust-gas aftertreatment device with a nitrogen oxide storage catalytic converter and a method for operating the exhaust-gas aftertreatment device, so that the release of H2S into the environment may be avoided.
The above and other beneficial objects of the present invention are achieved by providing a device and a method as described herein.
The exhaust-gas aftertreatment device according to the present invention is distinguished in that the exhaust gas which emerges from the nitrogen oxide storage catalytic converter may be fed to the SCR catalytic converter when the internal combustion engine is in a desulphating operating mode with a reducing exhaust-gas composition, in order for H2S which is formed during the desulphating to be removed. In this context, a SCR catalytic converter (SCR=selective catalytic reduction) should be understood to mean a catalytic converter which is generally employed to remove nitrogen oxides under oxidizing conditions. Catalytic converters of this type catalyze a selective reduction reaction in which nitrogen oxides are reduced using a reducing agent, e.g., ammonia (NH3), to form harmless N2.
Corresponding tests have shown that H2S which is fed to a SCR catalytic converter is reacted under reducing conditions to form SO2, with the result that the release of H2S to the environment during the desulphating of nitrogen oxide storage catalytic converters, which is carried out under reducing conditions, may be avoided. Since SO2 is far less odor-intensive than H2S, this effectively solves the problem of H2S being released into the environment. Moreover, the desulphating methods, which are often complicated precisely in order to avoid the release of H2S, may be greatly simplified. In this context, as is customary, the term reducing should be understood to mean a gas composition which, irrespective of any residual oxygen content, has an excess of constituents with a reducing action, such as hydrogen, carbon monoxide and/or hydrocarbons. The overall result, therefore, is an oxygen deficit with regard to the oxidation capacity of the abovementioned constituents with a reducing action.
The abovementioned benefits of preventing the release of H2S and/or NH3 into the environment also arise if further components for cleaning exhaust gases, such as a particle filter or a 3-way catalytic converter or an oxidation catalytic converter, are provided in the exhaust-gas aftertreatment device, provided that the SCR catalytic converter is arranged downstream of the nitrogen oxide storage catalytic converter.
In one example embodiment according to the present invention, the SCR catalytic converter contains vanadium pentoxide (V2O5) and/or tungsten oxide (WO3) and/or titanium dioxide (TiO2).
In an example embodiment of a method according to the present invention, the internal combustion engine is in a desulphating operating mode, the method including the steps of:
establishing a reducing exhaust-gas composition upstream of the nitrogen oxide storage catalytic converter,
releasing the sulphur which is bound in the nitrogen oxide storage catalytic converter, to form hydrogen sulphide (H2S),
feeding the hydrogen sulphide to an SCR catalytic converter which is arranged downstream of the nitrogen oxide storage catalytic converter in the exhaust-gas aftertreatment device,
and reacting the hydrogen sulphide in the SCR catalytic converter to form sulphur dioxide under reducing exhaust-gas conditions.
The desulphating operating mode of the internal combustion engine may, for example, include a purely rich-burn operating mode of the internal combustion engine with an air/fuel ratio (xcex) of 0.98 or less which is established for several seconds at simultaneously increased exhaust-gas temperatures of approximately 500xc2x0 C. or more. However, it is also possible to establish an oscillating lean/rich cyclical operation or to establish individual, short rich-burn operating phases, which are separated a number of times by lean-burn operating phases, for desulphating purposes. According to the method of the present invention, the desulphating may be carried out such that predominantly H2S is formed and is then converted into SO2 in the downstream SCR catalytic converter under reducing conditions, so that the odor problem during desulphating may be avoided. It is possible to eliminate the use of desulphating methods, which are particularly complicated in view of the need to avoid the release of H2S, for example only very slight enrichment of approximately xcex=0.99, optionally interrupted a number of times by lean-burn operating phases.
The likelihood of H2S being released during the desulphating of nitrogen oxide storage catalytic converters with a relatively high sulphur load is correspondingly more pronounced. To avoid the release of H2S as a result of considerable accumulation of sulphur, the desulphating may be carried out at relatively regular intervals, although this may be disadvantageous in terms of fuel consumption and driveability. However, the use of the method according to the present invention allows a relatively high accumulation of sulphur in the nitrogen oxide storage catalytic converter to be tolerated, and consequently the time intervals between desulphating operations may be selected to be correspondingly long, which may lead to considerable advantages with regard to operation of the internal combustion engine.